Explained: West Nile Virus; an unfamiliar disease, newly in focus in India

Abantika Ghosh
West Nile Virus is transmitted to humans through mosquito bites. (Representational)

On Sunday, a six-year-old boy died in Kozhikode from West Nile Virus infection. This has caused some alarm because the virus is relatively unknown in India. What is WNV, how did it reach Kerala, and is there a need to exercise extra caution?

The infection

According to the US Centers for Disease Control and Prevention, WNV is the leading cause of mosquito-borne diseases in the continental United States. It has so far been detected in almost all kinds of mosquitoes, not just the common three - Anopheles, Culex and Aedes - but also in lesser-known types such as Culiseta, Mansonia and Psorophora. It occurs during the summer months in the US, unlike other mosquito-borne diseases such as dengue, malaria and chikungunya, which occur all year round in India.

Cause & effect

The bite of an infected mosquito is the commonest mode of human infection. WNV can also spread through blood transfusion, from an infected mother to her child, or through exposure to the virus in laboratories. It is not known to spread by contact with infected humans or animals, or even when the infected animal is ingested, provided it has been adequately cooked.

Unlike other mosquito-borne diseases, WNV does not cause symptoms in everybody that contracts the virus. One in five people develops symptoms and requires medication. One in 150 people may go on to develop serious illness, or even die. However, WNV is dreaded because of the effects, often irreversible, that it has on the brain.

Ordinarily, the symptoms are the same as in any other viral fever - and include fever, headache, weakness, etc. But WNV can also cause nervous system symptoms such as stupor, disorientation, convulsion, tremors and loss of vision. Older people are more vulnerable, as are those with existing chronic conditions such as diabetes, hypertension, cancer, or those who have recently undergone organ transplants.

Where it is common

The virus is commonly found in Africa, Europe, the Middle East, North America and West Asia. The World Health Organization (WHO) documents that it was first isolated in a woman in the West Nile district of Uganda in 1937. It was identified in birds (crows and columbiformes like doves and pigeons) in the Nile delta region in 1953. Before 1997, WNV was not considered pathogenic for birds - but then, a more virulent strain caused the death in Israel of different bird species, presenting signs of encephalitis and paralysis. Human infections attributable to WNV have been reported in many countries in the World for over 50 years, the WHO says.

Should you panic?

No. There are no reports so far of the virus spreading to other parts of the country. Over the years, cases have been sporadic, and have occurred mostly in the Northeast. In the last three years, 12 cases have been reported outside the Northeast. In 2016, 15 cases were reported from the region; in 2017, 22 cases from Assam, Manipur, Tripura and Nagaland; in 2018, 22 cases from the entire region. That is why, says Dr Raman Gangakhedkar, head of Epidemiology and Communicable Diseases at the Indian Council of Medical Research (ICMR), the path the virus took en route to Malappuram, where the six-year-old victim is originally from, may be of academic interest, but has little relevance from a public health perspective. The fact that the numbers of WNV are far lower than those of the other better known mosquito-borne diseases like dengue etc., Dr Gangakhedkar explains, is that "the transmission is not very efficient, the replication rate of the organism is different". However, there is no marker or predictor of who will develop WNV symptoms after infection and who won't, and who will have a brain incursion and develop neurological symptoms, and who won't.

Precautionary measures

The Centre has sent a team from the National Centre for Disease Control (NCDC) to help Kerala health authorities deal with the West Nile Virus. The team includes Dr Ruchi Jain, RHO Thiruvananthapuram; Dr Suneet Kaur, assistant director, NCDC; Dr E Rajendran, entomologist, NCDC, Kozhikode; and Dr Binoy Basu, EIS Officer, NCDC. The Indian Council of Medical Research has also been alerted and a close watch is being maintained at central and state levels.